1. Field of the Invention
This invention relates to cameras, and more specifically to digital cameras and light-field cameras.
2. Description of the Related Art
FIG. 1 illustrates an exemplary conventional digital camera that employs a single objective lens 104 and a photosensor 108. A camera lens 104 is considered “fast” if the lens collects lots of light, thus making fast shutter speed possible. The F/number of a lens is a measure of how much light can be collected by the lens, where:F/number=(focal length)/(aperture diameter)
Quality fast objective lenses are desirable because they make photography in low lighting conditions possible, and also action photography possible. Fast general-purpose lenses are lenses with an F/number of F/1.8 or less. Telephoto lenses, for example for use in sports and wildlife photography, are considered fast even at F/2.8 because they are much harder to build at lower F/numbers. Even point-and-shoot cameras need relatively fast lenses if they are to be used in low light conditions in places where flash is inappropriate. However, quality fast lenses are hard to design and to build, and thus tend to be expensive. There are optical solutions to the problem of building fast lenses. However, those solutions get very expensive at low F/numbers due to the large angle of refraction far from optical axis at low F/numbers, which means correction for aberrations becomes difficult. Multiple high precision lens elements are needed. Also, those lens elements are bulky, making high-quality fast lenses big and heavy. The fact that high-quality objective lenses are typically actually a series of two or more individual lenses in series to provide correction for aberrations in individual lenses adds to both the bulk and the expense of the objective lenses.
Another approach is to increase photon efficiency of the photosensor (e.g., a charge-coupled device (CCD)). However, the industry has already reached near-optimal efficiency at acceptable prices. Also, at current efficiencies of 50% or more, achieving significant improvements is difficult. Even in theory efficiency of photosensors cannot improve more than two times better, as efficiency above 100% obviously cannot be reached.
Light-Field Cameras
Conventional cameras, as illustrated in FIG. 1, fail to capture a large amount of optical information. In particular, a conventional camera does not capture information about the location on the aperture where different light rays enter the camera. During operation, a conventional digital camera captures a two-dimensional (2D) image representing a total amount of light that strikes each point on a photosensor 108 within the camera. However, this 2D image contains no information about the directional distribution of the light that strikes the photosensor 108. Directional information at the pixels corresponds to locational information at the aperture.
In contrast, light-field cameras sample the four-dimensional (4D) optical phase space or light-field and in doing so capture information about the directional distribution of the light rays. This information captured by light-field cameras may be referred to as the light-field, the plenoptic function, or radiance. Radiance describes both spatial and angular information, and is defined as density of energy per unit of area per unit of stereo angle (in radians). A light-field camera captures radiance; therefore, images originally taken out-of-focus may be refocused, noise may be reduced, viewpoints may be changed, and other light-field effects may be achieved. Light-field cameras have been designed based on modifications to conventional digital cameras.
FIG. 2 illustrates an exemplary prior art light-field camera, or camera array, which employs an array of two or more objective lenses 110. Each objective lens focuses on a particular region of photosensor 108, or alternatively on a separate photosensor 108. This light-field camera may be viewed as a combination of two or more conventional cameras that each simultaneously records an image of subject 102 on a particular region of photosensor 108 or alternatively on a particular photosensor 108. The captured images may then be combined to form one image.
FIG. 3 illustrates an exemplary prior art plenoptic camera, another type of light-field camera, that employs a single objective lens and a microlens or lenslet array 106 that includes, for example, about 100,000 lenslets that is placed a small distance (˜0.5 mm) from a photosensor 108, e.g. a charge-coupled device (CCD). The raw image captured with a plenoptic camera 100 is made up of an array of small images, typically circular, of the main camera lens 108. These small images may be referred to as microimages. The lenslet array 106 enables the plenoptic camera 100 to capture the light-field, i.e. to record not only image intensity, but also the distribution of intensity in different directions at each point. Each lenslet splits a beam coming to it from the main lens 104 into rays coming from different “pinhole” locations on the aperture of the main lens 108. Each of these rays is recorded as a pixel on photosensor 108, and the pixels under each lenslet collectively form an n-pixel image. This n-pixel area under each lenslet may be referred to as a macropixel, and the camera 100 generates a microimage at each macropixel. The plenoptic photograph captured by a camera 100 with, for example, 100,000 lenslets will contain 100,000 macropixels, and thus generate 100,000 microimages of subject 102. By appropriately selecting a pixel from each macropixel, a conventional picture of subject 102 may be created from the microimages of subject 102 captured at the macropixels. Moreover, by mixing such images appropriately, images originally taken out-of-focus may be refocused, noise may be reduced, and other light-field effects may be achieved.